Active RIS has been recently proposed [1]}, [2]} to overcome the aforementioned practical issues of passive RIS by amplifying the reflected signal with low-cost hardware.
Unlike the conventional AF relay that requires power-hungry RF chains, the active RIS
directly reflects signals in an FD manner with low-power reflection-type amplifiers. It was shown that the active-RIS aided system is able to achieve higher rate than the passive RIS owing to the amplification gain at the RIS [1]}, [2]}. Moreover, the authors in [5]} considered the downlink/uplink communications separately and optimized the RIS placement for rate maximization with an
active or passive RIS. Simulation results showed that the active RIS should be deployed closer to the receiver with the less amplification power of the active RIS, while the passive RIS should be deployed close either the transmitter or receiver. However, the users at the back side of RIS still cannot be served by proposed active RIS.
The reflective property of RIS restricts the service coverage to only one side of the surface and the users locating behind RIS cannot be effectively served. To resolve this limitation, the novel concept of simultaneously transmitting and reflecting RIS (STAR-RIS) was proposed in [6]}-[7]}. In particular, the wireless signal impinging upon an element of a STAR-RIS is divided into two parts. One part is reflected to the users in front of the RIS, and the other is transmitted to the users behind the RIS. Thus, users at both sides of STAR-RIS can be served. Despite the above advantage on \(360^{\circ }\)  coverage, performance improvement of passive STAR-RIS is still limited due to severe signal attenuation.
